The decoders concerned are in particular decoder boxes (“Set-Top Boxes”) catering for the interface between an inlet for digital signals broadcast in real time by satellite, by cable or by terrestrial digital broadcasting (or DVB, standing for “Digital Video Broadcasting”) on the one hand, and an analog television on the other hand. Such a decoder box is a standalone apparatus. However, the invention applies also to integrated decoders built into a digital television, or to a digital read/record apparatus such as a DVD reader, a digital video cassette reader or the like.
A decoder box receives one or more programs in the form of a stream of compressed digital data, that is to say whose video frames (i.e. pictures) and audio frames are coded in such a way as to reduce the volume of the data broadcast. For example, this coding complies with the specifications of the MPEG-2 standard (ISO/IEC 13818-2) hereinafter the MPEG standard (“Motion Pictures Expert Group”). This standard defines a data compression technique for the coding of moving pictures and sound (in particular for digital television).
In the present description, the term “presentation”, when it is used in relation to a program, denotes the restoring of the decoded audio and video data, in audible and visible form respectively by a user. The term “playback” (or “reading”), used in relation to a program, more specifically denotes the decoding and the displaying of the program from a recording of the program on a hard disk. The term display is more particularly used to designate the presentation of the video frames.
The hard disk makes it possible to offer a number of functionalities, in particular the implementing of advanced modes of playback (or “trick modes”), that is to say a reading at a speed different from a nominal forward speed (×1), in particular fast forward and rewind at high speed or otherwise.
The difficulties encountered nowadays in the implementation of this latter functionality originate from the fact that the MPEG stream is, by nature, adapted to forward reading. Specifically, the pictures are transmitted in an order intended to facilitate forward decoding. For rewind reading, this order of transmission necessitates the availability of computational resources whose decoding rate is sufficient to carry out all the decoding operations required for decoding each picture. Failing this, a large storage capacity needs to be available for storing all the decoded pictures (i.e. the data of pictures after decompression) onwards of the start of a Group Of Pictures (or GOP) within the meaning of the MPEG standard, insofar as the pictures of the start of the GOP can again serve for the decoding of a next picture in the GOP. This storage capacity is supplied by frame memories, each of which is adapted for storing the data of a decoded picture.
In European Patent Application EP-A-0 735 780 there is proposed a technique for the rewind reading of an MPEG video stream using the same number of frame memories as for forward reading. This number is equal to three. Nevertheless, the method consists in skipping the B pictures, and in decoding a maximum of only three I or P pictures, then in going to the previous GOP (in the forward order of display) as soon as this number is reached.
International Patent Application WO 01/37572 proposes, for the rewind reading of an MPEG video stream, a technique making it possible to display all the pictures of a GOP. The technique consists in temporarily storing, in a minimum of four frame memories, the decompressed data of certain pictures which are useful in the subsequent decoding of several pictures to be displayed, rather than in recommencing the successive decoding operations from the beginning of the GOP. This document describes an exemplary GOP that can thus be decoded in its entirety by using four frame memories only. This example corresponds to the case of the reading of a program stored on a DVD, with GOPs containing a limited number of pictures. However, no guarantee is provided as to the result of the technique.
According what is needed is a method and system to over come the problems encountered in the prior art and to optimize the use of the frame memories available for the backward displaying of pictures of an MPEG video stream, in particular an MPEG video stream, stored on the hard disk of a decoder box.